1. Field of the Invention
The present invention relates to a method of forming silica glass into a desired shape by heating and pressing a synthetic silica bulk, and a forming apparatus thereof.
More particularly, the invention concerns a method of producing silica glass formed products with excellent optical characteristics, for example, suitable for optical members such as reticle (photomask) substrates, imaging optics, etc. in a high yield, by pressing synthetic silica bulks made of raw materials of silicon compounds such as silicon tetrachloride, silanes, organic silicons, and the like or synthetic silica bulks doped with a component to vary the index of refraction, such as Ge, Ti, B, F, Al, or the like, and a forming apparatus for carrying out the method.
2. Related Background Art
Reduction projection exposure systems (or photolithography systems) are mainly used for transfer of integrated circuit patterns such as ICs, LSIs, and so on. Requirements for projection optical systems used in the systems of this type are a wider exposure area with increase in integration of integrated circuits and higher resolving power throughout the entire exposure area. For enhancing the resolving power of the projection optical systems, it is conceivable to decrease the exposure wavelength to shorter wavelengths or to increase the numerical aperture (NA) of the projection optical systems.
The exposure wavelengths have been decreasing toward shorter wavelengths, e.g., from the g-line (436 nm) to the i-line (365 nm) and further to the KrF (248 nm) excimer laser and the ArF (193 nm) excimer laser. For further increase in integration of integrated circuits, research is now under way on methods for using a light source of the F2 (157 nm) excimer laser, X-rays, or an electron beam. Among these, the reduction projection exposure apparatus using the F2 excimer laser, which can be fabricated by taking advantage of the design concepts heretofore, is hastily moving into the limelight.
In general, in the case of optical glasses used as optical members of illumination optical systems or projection optical systems in the reduction projection exposure apparatus using the light sources of longer wavelengths than the i-line, the light transmittance decreases quickly in the wavelength region under the i-line and most optical glasses become untransmissive, particularly, to radiation in the wavelength region of not more than 250 nm. For that reason, silica glass, or single crystals of fluorides such as calcium fluoride, barium fluoride, and so on are usable as materials for lenses constituting the optical systems in the reduction projection exposure apparatus using the light source of excimer laser. These materials are essential materials to correction for chromatic aberration in the imaging optical system of excimer laser.
A reticle can be named as another significant element for printing circuits onto a wafer in the reduction projection exposure apparatus. The materials used for the reticle need to have transmission characteristics including endurance to the excimer lasers, of course, and another significant issue is thermal expansion due to heating of the substrate; therefore, the silica glass with good transmission characteristics and with a small coefficient of thermal expansion is used. Further important properties of the materials used for the reticle are chemical resistance and anti-etching characteristics thereof in view of the production processes.
A silica glass production method called a direct process is a method of mixing and burning a combustion gas (usually, oxygen gas) and a combustible gas (hydrogen gas or natural gas) in a silica glass burner, ejecting a silicon tetrachloride gas of high purity as a feed gas diluted with a carrier gas (oxygen gas, hydrogen gas, an inert gas, or the like), from the burner, making the feed gas react with water made by combustion of oxygen gas and hydrogen gas in the surroundings (to effect hydrolysis) to create silica glass particles, depositing the silica glass particles onto a target of a silica glass plate or the like undergoing rotation, swinging, and pulling-down motion, which is located below the burner, simultaneously melting the particles by heat of combustion of oxygen gas and hydrogen gas, and further vitrifying the deposits to obtain a silica glass ingot. This method permits the silica glass ingot to be obtained in relatively large diameter.
Another production method of silica glass ingot except for the direct process is a VAD (vapor-phase axial deposition) process making use of the technology used for production of optical fibers. This is a method of consolidating a porous silica glass preform by an atmospheric heat treatment.
The silica glass ingot fabricated by the production methods as described above is further cut into a glass block (synthetic silica bulk) having predetermined size and shape. Then this synthetic silica bulk is formed into a silica glass formed product of desired shape and size at high temperatures in a forming vessel of graphite and the formed product is used as an optical member such as the reticle or the like.
As one of such silica glass forming methods, Japanese Patent Application Laid-Open No. S56-129621 discloses the forming method of forming the bulk at the temperature of not less than 1700xc2x0 C. and in a helium gas atmosphere under the absolute pressure of 0.1 to 760 Torr in the graphite forming vessel and thereafter quenching the formed product down to 1100 to 1300xc2x0 C. Japanese Patent Application Laid-Open No. S57-67031 discloses the forming method in which the graphite forming vessel is of two or more-part split vertical structure. Further, Japanese Patent Publication of examined Application (KoKoku Publication) No. H04-54626 discloses the method of forming at 1600 to 1700xc2x0 C. by use of the graphite forming vessel having the structure for relaxing stress caused by the difference between thermal expansion coefficients of the silica glass and the forming vessel.
The conventional forming methods described above, however, had the problem that bubbles were created in the synthetic silica bulk (silica glass) in the middle of pressing at high temperatures and a lot of bubbles remained in the silica glass formed product after the forming. This silica glass formed product with many bubbles remaining inside cannot be used as an optical member. Particularly, in the case of the synthetic silica bulks doped with the component to vary the refractive index, such as Ge, Ti, B, F, Al, or the like, because the viscosity thereof is high, a lot of bubbles tend to remain inside the silica glass formed product after the forming.
In general, in cases wherein the synthetic silica bulk (silica glass) is formed by pressing in the forming vessel, there occurs great difference in shrinkage between the synthetic silica bulk and the forming vessel during cooling down to the room temperature after the forming at high temperatures, because the constitutive materials of the silica glass bulk and the forming vessel have their respective coefficients of thermal expansion largely different from each other. For that reason, in the conventional methods, unwanted stress was exerted on the synthetic silica bulk and the forming vessel, so as to result in cracking of the silica glass formed product formed by pressing, or even breakage of the forming vessel in certain cases.
There also arises the problem that the constitutive materials of the synthetic silica bulk and the forming vessel react with each other at high temperatures. For example, where the graphite forming vessel is used as a forming vessel, the synthetic silica bulk reacts with graphite at high temperatures to create silicon carbide. Therefore, the surface of the silica glass formed product after the forming became rough, depending upon the forming temperatures, and cracking occurred from the rough surface in some cases.
An object of the present invention is, therefore, to provide a method of producing silica glass formed products with excellent optical characteristics free of the remaining bubbles and the cracking and suitable for the optical members such as the reticle substrates, the imaging optical systems, and so on in a high yield, by pressing the synthetic silica bulks made of the raw materials of silicon compounds such as silicon tetrachloride, silanes, organic silicons, and the like and the synthetic silica bulks doped with the component to vary the refractive index, such as Ge, Ti, B, F, Al, and so on; and a forming apparatus for carrying out the method.
The inventors have conducted intensive and extensive studies in order to achieve the above object and found out that the above problems could be solved by interposing an elastic member with such permeability as to absorb a pressure difference appearing between the synthetic silica bulk to be processed and the forming vessel, between the synthetic silica bulk and the forming vessel, thus accomplishing the present invention.
Namely, a forming method of silica glass according to the present invention is a silica glass forming method of pressing a synthetic silica bulk having at least a set of opposed surfaces, on the surfaces under a high temperature condition by pressing means, wherein an elastic member with permeability is placed between the pressing means and said surfaces of the synthetic silica bulk pressed by the pressing means and wherein the synthetic silica bulk is pressed through the elastic member by the pressing means.
The conventional methods had the problem that bubbles appeared in the synthetic silica bulk during the pressing at high temperatures and many bubbles remained in the silica glass formed product after the forming. For solving this problem, the inventors judged that it was difficult to suppress the occurrence of bubbles itself, because it was mentioned that a temperature slope in the synthetic silica bulk during the pressing at high temperatures contributed to the occurrence of bubbles and because it was difficult to control temperature distribution uniform in the synthetic silica bulk during the pressing at high temperatures. In view of this judgment, the inventors conducted studies on how to quickly dissipate the evolving bubbles to the outside of the synthetic silica bulk and to the outside of the forming vessel, and accomplished the present invention.
Describing in more detail, in the present invention as against the conventional methods, the elastic member with permeability is interposed between the synthetic silica bulk and the forming vessel whereby the bubbles evolving inside the synthetic silica bulk during the forming can be quickly dissipated to the outside of the synthetic silica bulk, and thus the bubbles remaining inside the silica glass formed product after the forming can be reduced to a sufficiently small amount.
In the conventional methods, since the constitutive materials of the synthetic silica bulk and the forming vessel had their respective coefficients of thermal expansion greatly different from each other, there occurred the great difference in shrinkage between the synthetic silica bulk and the forming vessel, which caused the cracking of the silica glass formed body after the pressing or even the breakage of the forming vessel in some cases. In contrast to it, in the present invention, the elastic member with permeability is interposed between the synthetic silica bulk and the forming vessel as described above, whereby the elastic member with permeability functions as a medium for absorbing compressive stress and tensile stress appearing between the synthetic silica bulk and the forming vessel, which can fully suppress the occurrence of stress between the two members during the cooling process after the pressing.
Specifically, when the forming vessel is a graphite forming vessel, the coefficients of linear expansion of silica glass (synthetic silica bulk) and graphite are 5xc3x9710xe2x88x926 to 6xc3x9710xe2x88x926/xc2x0C. and 2xc3x9710xe2x88x924 to 6xc3x9710xe2x88x924/xc2x0C., respectively, and the degree of shrinkage of silica glass is small while that of graphite is large during the cooling process after the pressing at high temperatures. As a consequence, compressive stress acts on the synthetic silica bulk and the tensile stress on the graphite. This facilitated occurrence of breakage of the graphite forming vessel and also caused the cracking of the silica glass formed product due to the compressive stress in the conventional methods. Particularly, in cases wherein the component to vary the refractive index is added, since there is also large difference in viscosity between the synthetic silica bulk and the graphite in addition to that in the coefficient of thermal expansion, it increases the possibility of causing the cracking of the silica glass formed product after the forming and the breakage of the graphite forming vessel more than that with the ordinary synthetic silica bulks. In contrast to it, the present invention can fully suppress the cracking of the silica glass formed product and the breakage of the graphite forming vessel, by interposing the elastic member having the permeability and functioning as a medium for absorbing the compressive stress and tensile stress appearing between the synthetic silica bulk and the forming vessel, between the synthetic silica bulk and the forming vessel.
Further, in the present invention, direct contact between the synthetic silica bulk and the forming vessel is avoided by the elastic member with permeability interposed between the synthetic silica bulk and the forming vessel, and thus it can prevent the reaction between the constitutive materials of the synthetic silica bulk and the forming vessel, which occurred during the pressing in the conventional methods.
In the silica glass forming method of the present invention, the elastic member may be a woven or nonwoven fabric of carbon fiber.
Since the woven or nonwoven fabric of carbon fiber structurally has adequate permeability, interposition of it between the synthetic silica bulk and the forming vessel permits the bubbles evolving inside the synthetic silica bulk during the forming to diffuse in this woven or nonwoven fabric of carbon fiber and be dissipated to the outside more quickly. Therefore, bubbles remaining in the silica glass formed product after the forming can be reduced to a sufficiently small level.
Further, in the silica glass forming method of the present invention, the elastic member may be a woven or nonwoven fabric of ceramic fiber.
Just like the woven or nonwoven fabric of carbon fiber, the woven or nonwoven fabric of ceramic fiber also structurally has adequate permeability. Therefore, bubbles remaining in the silica glass formed product after the forming can be reduced to a sufficiently small level.
In the silica glass forming method of the present invention, it is preferable that the pressing by the pressing means be carried out under a pressure not less than the atmospheric pressure and in an inert gas atmosphere.
When the pressing is carried out in the inert gas atmosphere in this way, the constitutive materials of the synthetic silica bulk and the forming vessel can be prevented more effectively from reacting with each other. For example, where the forming vessel is the graphite forming vessel, the above method can more effectively prevent the trouble that the synthetic silica bulk and the graphite forming vessel react with each other to form silicon carbide.
Further, in the silica glass forming method of the present invention, it is preferable that the pressing by the pressing means be carried out at the temperature of 1750-1850xc2x0 C. and for 10-60 minutes, preferably 10-30 minutes.
The trouble of the reaction between the constitutive materials of the synthetic silica bulk and the forming vessel can be prevented more effectively by setting the temperature during the forming to 1750-1850xc2x0 C. and the retention time at that temperature to 10-60 minutes, preferably 10-30 minutes as described. The conventional methods involved the problem that the synthetic silica bulk crystallized in the temperature region of 1400-1600xc2x0 C., but the crystallization of the synthetic silica bulk can be restrained by setting the temperature during the forming to 1750-1850xc2x0 C. and the retention time at that temperature to 10-60 minutes, preferably 10-30 minutes as described above. When it becomes feasible to prevent the reaction between the constitutive materials of the synthetic silica bulk and the forming vessel and restrain the crystallization of the synthetic silica bulk, it also becomes feasible to prevent occurrence of the unevenness and cracking in the surface of the silica glass formed product after the forming. Further, execution of the pressing at the temperature and in the range of the retention time as described permits the bubbles evolving inside the synthetic silica bulk to be reduced to a sufficiently small amount.
On the other hand, if the temperature during the pressing is less than 1750xc2x0 C. there will be a tendency toward more reduction in the yield due to the crystallization of the synthetic silica bulk. If the temperature exceeds 1850xc2x0 C. there will be a tendency toward creation of more bubbles in the synthetic silica bulk due to evaporation of silica glass. If the pressing time is less than ten minutes the formability will degrade. If it exceeds 30 minutes there will be increasing tendencies to cause the crystallization of the synthetic silica bulk and the creation of bubbles in the synthetic silica bulk.
In the silica glass forming method of the present invention, it is preferable that the synthetic silica bulk have a side face connecting outer edges of the set of opposed surfaces and that an elastic member be placed between this side face and the pressing means. This makes it feasible to dissipate the bubbles appearing in the synthetic silica bulk during the pressing to the outside of the synthetic silica bulk more surely, to prevent the occurrence of compressive stress and tensile stress between the synthetic silica bulk and the forming vessel during the pressing more surely, to prevent the reaction between the constitutive materials of the synthetic silica bulk and the forming vessel during the pressing more surely, and to prevent the breakage of the forming vessel after the pressing more surely.
Further, in the silica glass forming method of the present invention, it is preferable that the synthetic silica bulk have a side face connecting outer edges of the set of opposed surfaces and that a width of the elastic member placed between this side face and the pressing means be not more than a thickness after the forming of the synthetic silica bulk. This permits surer prevention of breakage of the silica glass formed product after the forming and the elastic member.
In the silica glass forming method of the present invention, it is preferable that bulk density of the elastic member be 0.1-0.5 g/cm3.
When the bulk density of the elastic member is 0.1-0.5 g/cm3, the elastic member has better porosity (permeability) and elasticity. Therefore, the bubbles appearing inside the synthetic silica bulk can be eliminated adequately to the outside during the pressing, which can prevent the trouble of the bubbles remaining inside the silica glass formed product after the pressing with more certainty.
However, if the bulk density of the elastic member is less than 0.1 g/cm3, the strength of the elastic member will be inadequate and it will accelerate the tendency to arouse trouble of breakage of the elastic member during the pressing. If the bulk density is over 0.5 g/cm3 on the other hand the elasticity of the elastic member will be too low to sufficiently absorb the compressive stress and tensile stress due to the difference in shrinkage between the synthetic silica bulk and the forming vessel, so as to accelerate the tendency to make trouble of breakage of the synthetic silica bulk or the forming vessel.
Further, in the silica glass forming method of the present invention, it is preferable that a thickness of the elastic member be 1-20 mm.
When the thickness of the elastic member is 1-20 mm, the elastic member has sufficient elasticity and porosity (permeability). Therefore, it can adequately absorb the compressive stress and tensile stress due to the difference in shrinkage between the synthetic silica bulk and the forming vessel during the pressing and adequately eliminate the bubbles appearing inside the synthetic silica bulk to the outside.
However, if the thickness of the elastic member is less than 1 mm the strength of elastic member will be insufficient and it will accelerate the tendency to cause trouble of breakage of the elastic member during the pressing. If the thickness is over 20 mm on the other hand, handling will become poor and the elasticity of the elastic member will be insufficient, which will, in turn, enhance the tendency to give rise to the trouble of the bubbles remaining inside the silica glass formed product after the pressing.
A silica glass forming apparatus of the present invention is a forming apparatus of silica glass comprising a forming vessel for accommodating a synthetic silica bulk having at least a set of opposed surfaces, pressing means for pressing said surfaces of the synthetic silica bulk under a high temperature condition, and a heating device for heating the forming vessel, wherein an elastic member with permeability is placed between the pressing means and said surfaces of the synthetic silica bulk pressed by the pressing means and wherein the synthetic silica bulk is pressed through the elastic member by the pressing means.
Since the silica glass forming apparatus of the present invention comprises the elastic member with permeability placed between the pressing means and the surfaces of the synthetic silica bulk pressed by the pressing means in accordance with the aforementioned silica glass forming method of the present invention as described above, it becomes feasible to more certainly dissipate the bubbles appearing in the synthetic silica bulk during the pressing to the outside of the synthetic silica bulk, to more certainly prevent the compressive stress and tensile stress from appearing between the synthetic silica bulk and the forming vessel during the pressing, and to more certainly prevent the constitutive materials of the synthetic silica bulk and the forming vessel from reacting with each other during the pressing. Therefore, the silica glass forming apparatus of the present invention is able to produce the silica glasses with excellent optical characteristics free of the remaining bubbles and the cracking and, for example, suitable for the optical members such as the reticle substrates, the imaging optical systems, etc. in a high yield.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.